1. Field of the Invention
The invention relates to gas turbine vanes and more particularly, methods for welding vane containment caps to gas turbine vane inner shroud inboard surfaces for enhanced in-service crack resistance along the welds.
2. Description of the Prior Art
As explained in greater detail in U.S. Pat. No. 5,098,257,, which is incorporated herein by reference, a portion of the annular gas flow path in the turbine section of a gas section is formed by a plurality of vane segments circumferentially arrayed around the rotor. Each vane segment is comprised of an inner and an outer shroud, which together form the boundaries of the gas flow path, and one or more vanes. In order to insure that the material forming the vane segments is not overheated, thereby compromising its strength, the vane segments of modern gas turbines are cooled with air bled from the compressor section. This cooling air is often supplied to both the inner and outer shrouds, from which it is distributed throughout the vane segments. In order to utilize this cooling air effectively, external structures are formed on the vane segment shrouds to contain and distribute the cooling air. Typically, these structures are attached to the surfaces of the shrouds opposite the surfaces exposed to the hot gas flowing through the turbine section.
General structure of an exemplary known gas turbine Row 1 vane 20 and downstream Row 1 blade 22 are shown in FIGS. 1 and 2. Vane segments 24 are encased by a blade ring 26. Also, the vane segments 24 encircle an inner cylinder structure 28. The inner cylinder structure comprises a ring 30 affixed to a rear flange 32 of the inner cylinder 28. During operation, hot gas (arrow 34) from the combustion section (not shown) is directed to flow over the vane segments 24. The flow of hot gas 34 is contained between the outboard surface 36 of the inner shroud 38 and the inboard surface 40 of the outer shroud 42. As explained in greater detail in U.S. Pat. No. 5,098,257, which is incorporated herein by reference, cooling air is bled from the compressor section, thus bypassing the combustors, and is supplied to the inner 38 and outer 42 shrouds. A portion of the cooling air flows through one or more holes in the blade ring 26, from whence it enters the outer shroud 42 into the vane segment 24 and flows through the vane air foil. Some cooling air discharges into the hot gas 34 through holes (not shown) in the walls of the airfoil portion of the vane segment 24. A portion of the cooling air 10 flows through holes formed in the inner shroud 38 and is contained by a pan-like left containment cap 44. The left containment cap 44 is affixed to the inboard surface 46 of the inner shroud 38. As a structural lug 48 emanates radially inward from the inboard surface 46 of the inner shroud 38, the left containment cap 44 serves to prevent leakage of cooling air to the turbine section by being welded to the inner shroud and the lug. Cooling air in the left containment cap 44 flows through an opening in the lug 48 (not shown) and enters a right containment cap 50, which also contains the cooling air. The right containment cap 50 is also welded to the inboard surface 46 of the inner shroud 38, in similar fashion as the left containment cap 44. The containment caps 48 and 50 thus allow cooling air to circulate on the inboard side of the inner shroud 38 but constrain the cooling air from venting radially inwardly toward the centerline of the turbine.
As shown in the enlarged FIG. 2 and in the FIG. 3 perspective view, the rim 52 of the right containment cap 50 and the rim 54 of the left containment cap 44 are welded to the inboard surface 46 of the inner shroud 38 by weld bead 56. Further discussion of containment cap welding herein applies equally to either the left containment cap 44 or the right containment cap 50, but for simplicity future reference will focus on the right containment cap structure 50 or “the containment cap 50”. Referring to FIG. 4, the known weld bead 56 is a built-up fillet weld formed on the peripheral boundaries of the adjacent rim surface 52 of the right containment cap 50 and the inboard surface 46 of the inner shroud 38. The adjacent surfaces of the rim 52 and the inboard surface 46 are not bonded to each other, leaving a crevice between those surfaces.
The fillet weld 56 is susceptible to cracking while the turbine engine is in service. The weld 56 initiated crack can propagate from the inboard side of the weld, where the crevice exists between the rim 52 and inboard surface 46, through the rest of the containment cap 50. The crack so formed may lead to an unplanned service outage. Less severe crack propagation in the fillet weld 56 and containment cap 50 may not cause a service outage and will be found during scheduled routine maintenance inspection when the engine is being repaired. Unfortunately, the fillet weld 56 and/or propagated containment cap 50 cracks may be too severe for service repair, leading to scrapping an expensive vane segment 24. As shown in the FIG. 5 photograph cross section of an actual known fillet weld crack, it appears that the fillet weld between the cap 50 and the vane inner shroud inboard surface 46 creates a stress riser, leading to the crack initiation. Substitution for different weld filler materials to form the fillet weld 56 has not resolved the crack initiation problem.